During the production of polyamide 6 (PA6) granules by means of polycondensation from lactams, the melt obtained in the VK tube, after cooling (normally with warm water which contains small amounts of lactam), is processed to granules or chips either by strand or underwater granulation. The water required for the granulation is also referred to as cutting water. The temperature, amount and permissible concentration of lactam in the cutting water required in a particular case is governed in this case on the one hand by the granulating method used and on the other hand also by the desired appearance of the PA6 chips.
Since the granulated PA6 chips contain amounts of up to 11% lactam and oligomers, which are at least partially water-soluble, the content of lactam and oligomers in the cutting water would increase in concentration constantly in the course of the granulation, which could lead to problems with the granulation and even to failure of the granulation, for example because of increased foam formation and/or to blockage of the filters in the cutting water circuit. The elimination of such faults can take several hours in some cases and, even after the fault has been eliminated, variations in quality persist for several hours of production. These variations in quality generally have the result that, during the mentioned period of time, no granules of the desired quality can be produced or only a so-called B quality can be produced, which can be separated out and sold only at a reduced price. In order to avoid such faults, a specific stream of pure demineralized water is generally fed into the cutting water stream continuously or batchwise, in order to limit or adjust the content of dissolved lactam and oligomers. The respective permitted limits for lactam and oligomers in the cutting water vary according to the granulating method and the desired cutting water temperature. Empirical values for the content of lactam and oligomers in the cutting water are here approximately from 1.0 to 3.0% by weight for lactam and oligomers. The excess water in the cutting water circuit is removed by means of overflow control or automatic level control and fed to the lactam/water recycling system (evaporation). The set amount of demineralized water which is fed into the cutting water circuit must accordingly additionally be processed in the evaporation and thus increases the consumption of heat energy (generally heating steam). It is thus necessary, depending on the equipment present and the operational experience, to find the optimum setting point for the addition of the demineralized water after a certain time. The amounts of demineralized water are frequently set too low in order to save heating steam, but this leads sooner or later to the mentioned problems in the granulation.
The oligomers and monomers of the raw material lactam (and oligomers of the raw materials required for the production of copolymers) which remain in the melt in the polycondensation of polyamide 6 (or polyamide 6 with copolymer content<30%) must be separated off in an extraction unit arranged downstream. The content of monomers and oligomers is in most cases above 6% by weight. The separation is generally carried out by means of a one- or two-stage extraction process. The cooled polymer comminuted to granules is thereby guided countercurrently to an extraction liquid (generally water with different lactam contents). The extraction liquid thereby takes up the monomers and oligomers (extractable substances) from the polymer granules, the total content of extractable substances in the granules can be reduced to far below 1% by weight. The extraction liquid, which remains in the polymer in place of the extractable substances, must be removed in a subsequent drying step. The flow guidance is generally such that the granules in a trickle bed are guided from top to bottom through the extraction vessel, wherein the extraction liquid is guided from bottom to top. A uniform flow of granules and extraction liquid is to be ensured by means of built-in components.
On account of the different solubility of monomers and oligomers in the extraction liquid, the extraction is frequently carried out in two (or more) steps. In a first stage, the oligomers are separated off with a suitable extraction liquid, mostly water having a lactam content of from 5 to 50% by weight. In one (or more) further extraction stages, the monomers and the residual oligomers are removed from the polymer by means of a suitable extraction liquid, mostly water having a lactam content of less than 0.5% by weight.
From U.S. Pat. No. 8,541,540 B2 there is known a continuous process in which the water already used in the extraction stage is used instead of fresh water for the granulation, so that the energy consumption of the process can be reduced.
The temperature to which the extraction liquid is heated has a very great influence on the result of the extraction. Higher temperatures show better extraction results. For reasons of cost, the extraction apparatuses are mostly in the form of pressureless apparatuses. The maximum adjustable inlet temperature of the extraction liquid is accordingly the boiling temperature respectively corresponding to the hydrostatic pressure.
The extraction liquid is conventionally brought to the desired temperature once by means of internal or external heat registers, wherein the maximum adjustable temperature is the boiling temperature corresponding to the respective hydrostatic column in the extraction apparatus. This is fed in at the bottom of the extraction apparatus. However, this method does not offer the possibility of being able to adjust the extraction liquid to its respective boiling temperature over the entire length of the extraction apparatus. Especially when a multi-stage extraction must be used in order to achieve the required residual extract content of significantly less than 1.0% by weight, the inlet temperature of the pre-extracted granules is close to the boiling temperature of the extraction liquid. The transport liquid is separated off at the top of the extraction reactor. Owing to the high temperature of the granules, the extraction liquid is scarcely cooled in the extraction column. For this reason, additional cooling of the extraction liquid is necessary.
Therefore, a need exists for a process for producing polyamides in which additional cooling of the extraction liquid can be avoided and an economical process is thus permitted.